The present invention relates to a method and apparatus for reducing vortex-induced-vibrations ("VIV") and, more particularly, reducing VIV in marine environments by the use of fairings.
Production of oil and gas from offshore fields has created many unique engineering challenges. One of these challenges is dealing with effects of currents on fixed cylindrical marine elements. Such marine elements are employed in a variety of applications, including, e.g., subsea pipelines; drilling, production, import and export risers; tendons for tension leg platforms; legs for traditional fixed and for compliant platforms; other mooring elements for deepwater platforms; and, although not conventionally thought of as such, the hull structure for spar type structures. These currents cause vortexes to shed from the sides of the marine elements, inducing vibrations that can lead to the failure of the marine elements or their supports.
For short cylindrical elements that are adjacent convenient means for secure mounting, the marine elements and their supports can be made strong enough to resist significant movement by the forces created by vortex shedding. Alternatively, the marine element could be braced to change the frequency at which the element would be excited by vortex shedding.
However, strengthening or bracing becomes impractical when the application requires that the unsupported segments of marine element extend for long runs. Deepwater production risers, drilling risers, platform export risers, import risers bringing in production from satellite wells, tendons for tension leg platforms, and other conduits for produced fluids and deepwater mooring elements formed from tubular goods are typical of such applications. These pipes and tubular goods serve as marine elements in applications that are difficult or impossible to brace sufficiently to satisfactorily control vibration induced by vortex shedding. Subsea pipelines traversing valleys on the ocean floor for extended, unsupported lengths and spar hulls moored at the end of long tethers and/or mooring lines provide additional examples.
Shrouds, strakes and fairings have been suggested for such applications to reduce vortex induced vibrations. Strakes and shrouds can be made to be effective regardless of the orientation of the current to the marine element. But shrouds and strakes are generally less effective than fairings and generally materially increase the drag acting on the marine element. Fairings are generally very effective in reducing vibrations due to vortex shedding, and also reduce drag forces on the marine element.
Until recently, however, fairings have been thought to be effective only if 1) the fairing had a relatively long chord to thickness ratio; 2) the relative direction of the current was close to parallel with the orientation of the fairing; and 3) the (small) diameter of the marine element and design case for current velocity were such as to combine for a relatively low Reynolds number.
U.S. Pat. No. 5,410,979 represents a significant breakthrough in providing for fixed, short fairings. Even so, short fairings having a maximum ratio of length to width of from 1.5 to as low as about 1.25 still drive cost and otherwise limit application. This length to width ratio, as a practical matter, translate to about a 1.20 minimum chord to thickness ratio for short fairings. Further, unexpected results documented in the contemporaneous filed patent application (Provisional Ser. No. 60/031,271 filed Nov. 15, 1996) of Donald W. Allen and Dean L. Henning for Ultrashort Fairings for Suppressing Vortex-Induced Vibrations show effective fairing designs for offshore applications with fairings having a chord to thickness ratio of 1.20 down to about 1.10.
Further, short and ultrashort fairings have been demonstrated effective with large diameter marine elements, such as larger diameter drilling risers and even spar hulls, which were conventionally considered unsuitable for fairings although subject to VIV problems as moored at the distal end of long tethers and/or mooring lines which provide no suitable means for bracing support.
U.S. Pat. Nos. 4,389,487 and 4,474,129 disclose fairings for use with subsea pipes and risers which are provided with means to permit the fairing to rotate around the pipe or riser as would a weathervane in order to maintain an orientation presenting the fairing parallel to the current. Accommodating this constraint results in an expensive apparatus.
Further, the subsea environment in which the fairings must operate renders likely the rapid failure of the rotational elements. Traditional fairings present a chord to thickness ratio greater than two and present a very serious problem should corrosion or marine growth cause the rotational elements to seize up. Such a failure of the fairing to rotate would cause excessive drag forces on the marine element should the current shift and no longer align with the "frozen" fairing. As a result, rotatable fairings have, in actual practice, been limited to drilling riser applications in which the risers (together with fairing mounted thereon) are frequently and routinely retrieved and not left in service for extended periods.
However, the reduced drag of misaligned short and ultrashort fairings manages this risk and such fairings have now been shown to have practical application, even if fixed. Further, the effectiveness of such short and ultrashort fairings can extend to current angles relative to the fairing (angle of attack) of up to a range approaching +/-60 degrees and somewhat higher for ultrashort fairings.
In some regions, ocean currents are sufficiently constant in direction and intensity such that normally occurring fluctuations are of little significance to fatigue life design and VIV which is controlled for currents having an angle of attack within such a window is satisfactory. However, in other regions, patterns of prevailing currents show greater variation with extended periods and higher currents across a greater deviation. There it is important to accommodate a wider range of current angles in order to avoid significant VIV precipitated degradation to fatigue life.
Thus, despite recent breakthroughs, there remains a need for economical fairings that are well adapted to applications for reducing VIV on marine elements in areas of expanded directional current fluctuations.
An advantage of the present invention is to provide an array of fixed fairings of sufficiently limited drag to as to enable an array of fixed fairing orientations to be simultaneously deployed on a single marine element, broadening the environmental conditions that will meet adequate VIV suppression while retaining the benefits of fixed fairings.